Metal-air batteries include an air permeable cathode and a metallic anode separated by an aqueous electrolyte. For example, in a zinc-air battery, the anode contains zinc, and during operation, oxygen from the ambient air is converted at the cathode to hydroxide, zinc is oxidized at the anode by the hydroxide, and water and electrons are released to provide electrical energy. Metal-air batteries have a relatively high energy density because the cathode of a metal-air battery utilizes oxygen from ambient air as a reactant in the electrochemical reaction rather than a heavier material such as a metal or metallic composition. This results in a relatively lightweight battery.
Both primary and secondary metal-air batteries have been developed. A rechargeable metal-air battery is recharged by applying voltage between the anode and cathode of the metal-air battery cell and reversing the electrochemical reaction. Oxygen is discharged to the atmosphere through the air permeable cathode.
Metal-air battery cells are often arranged in multiple cell battery packs to provide a sufficient amount of power output. In addition, it is often necessary to expose the air cathodes of the cells to a continuous flow of air at a flow rate sufficient to achieve the desired power output. Such an arrangement is shown in Cheiky U.S. Pat. No. 4,913,983 wherein a fan is used to supply a flow of air to a pack of metal-air battery cells.
One problem with metal-air batteries is that the ambient humidity can cause the metal-air battery to fail. Equilibrium vapor pressure of the metal-air battery results in an equilibrium relative humidity that is typically about 40 percent. If ambient humidity is greater than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will absorb water from the air through the cathode and fail due to a condition called flooding. Flooding may cause the battery to burst. If the ambient humidity is less than the equilibrium relative humidity value for the metal-air battery, the metal-air battery will release water vapor from the electrolyte through the air cathode and fail due to drying out. In most environments where a metal-air battery is used, failure occurs from drying out.
In electrochemical cells and in oxygen depolarized cells in particular, heat is generated at the cathode, anode, and electrolyte as ohmic losses and electrode polarization potential on both charge and discharge. In high power batteries, this waste heat can, at the extreme, cause the water component of the electrolyte to boil and can initiate rapid decomposition of battery components such as separators.
The problem is especially acute in oxygen depolarized cells in that the oxygen diffusion electrode(cathode) typically passes water vapor as freely as oxygen due to the similar size and polarization of diatomic oxygen and gaseous water molecules. Thus, as air is supplied to such batteries on discharge, or vented on recharge (in the case of rechargeable batteries), water vapor freely passes through the cathode as well. If the battery electrolyte temperature rises above ambient, then the water vapor pressure at the internal surface of the gas diffusion electrode will exceed the ambient water vapor pressure, and the cell will lose water to the environment by evaporation. This can be made up by water addition to the cell but, in many applications, freedom from maintenance or low maintenance is a requirement so minimizing water loss due to cell internal heating is of critical importance.
The traditional geometry for high power metal-air batteries and fuel cells made of multiple cells is to place the anode structure between two cathodes in each cell as per Katsouis et. al. U.S. Pat. No. 3,518,123, Chodosh U.S. Pat. No. 3,960,600 and Turley et. al. U.S. Pat. No. 4,927,717 in metal-air batteries, and Truitt U.S. Pat. No. 3,458,357, Joy U.S. Pat. No. 4,560,626 and Shimizu et. al. U.S. Pat. No. 4,562,123 in fuel cells. When cells of this construction are stacked as per Petix U.S. Pat. No. 3,682,705, Sauer et. al. U.S. Pat. No. 4,115,626, Niksa et. al. U.S. Pat. No. 4,693,946 and others, the air chambers between adjacent cells serve both to provide reaction oxygen and to provide cell cooling by heat conduction to the air stream and evaporative cooling due to water vapor diffusion from the battery cell or fuel cell cathodes.
Another attempt to deal with heat generation, by increasing the air flow rate over the air cathode to cool the metal-air battery, is disclosed in Terry et. al. U.S. Pat. No. 3,395,047. However, the increase in the air flow rate over the air cathode increases the rate of vaporization of the water and offsets the decrease in water loss from the cooling effect.
Cheiky U.S. Pat. No. 4,894,295 presents a cell with only one cathode in which an integral cathode air chamber and support provide air ducting to the cathode. The anode is a separate surface typically comprising a metal current collector and plastic outer shell. Such cells are typically operated in an air manager as per Cheiky U.S. Pat. No. 4,913,983 in which the air supply is regulated in response to the power demand. At power levels producing above 20 ma per square inch of anode, the internal temperature rise of such cells can cause sufficient water transport from the electrolyte into the cathode air stream to adversely affect battery operating life.
Another problem with a metal-air battery is that contaminants in the air such as carbon dioxide, cigarette smoke, and sulfides decrease the battery output. For example, carbon dioxide reacts with the metal hydroxide formed by the reaction between the anode and electrolyte. The reaction between carbon dioxide and the metal hydroxide forms a metal carbonate compound that interferes with the electrochemical reaction. The exposure of metal-air battery cells to contaminants is increased if the air flow rate over the cathodes is increased for cooling.
Drying out and flooding are even greater problems for secondary metal-air batteries than for primary metal-air batteries. Although ambient humidity may not be sufficient to flood or dry out a battery after a single cycle, cumulative water gain or loss from a series of discharge and charge cycles can cause premature failure of a secondary metal-air battery,.
Accordingly, there is a need for an air manager system for a metal-air battery that minimizes the effects of ambient humidity, heat generation and contaminants on the useable life of the battery.